1. Field of the Invention
The present invention relates to composite materials having a low coefficient of thermal expansion (CTE). In particular, the invention relates to metal matrix composites (MMC""s) possessing high thermal conductivity and low, preferably near-zero CTE""s.
2. Discussion of Related Art
It has been known for a long time to add fibrous reinforcement to metals to increase mechanical properties such as specific strength and specific stiffness. One of the early such reinforcements was carbon or graphite fiber, produced from polymeric precursors. The resulting composite material offered double or triple the strength or stiffness compared to the bulk, unreinforced metal. Processing was difficult, however, as the metals either tended not to wet the carbon fibers, or reacted with the carbon. Considerable energy has been devoted to developing ways to preserve the chemical and physical integrity of the fibers while rendering them more chemically compatible with the metal matrix.
Carbon fibers can be manufactured with high degrees of anisotropy. The graphite form of carbon in particular features a hexagonal crystallographic structure, with the covalent bonds within the {001} planes being strong, and the bonds between the {001} planes consisting of weak van der Waals bonds. It is possible to preferentially align the crystallographic planes in a graphite fiber such that the {001} planes tend to be parallel to the graphite fiber axis. By increasing the relative amount of covalent bonds in the fiber axis direction, a fiber possessing high strength and high elastic modulus in the direction of the fiber axis is produced. An interesting phenomenon that accompanies the alignment of the high strength, high modulus directions is that this particular direction also possesses a (rare) negative CTE. Thus, instead of expanding upon heating like most materials, these fibers actually shrink in the axial direction. In the radial direction of such fibers, however, the strength and elastic moduli are relatively low and the CTE is positive and relatively high.
By incorporating parallel arrays of such fibers into a positive CTE isotropic matrix, a composite material having a high modulus and a zero or near-zero CTE in the axial direction of the fibers can be produced. In the direction transverse to the fiber axes, the modulus would be relatively low and the CTE would be relatively high. Because of the axial stiffness, the properties of the composites tend to be dominated by the axial properties.
The degree of anisotropy can be reduced by distributing the fiber orientations. One technique for accomplishing this is to arrange the fibers in parallel within a thin sheet or xe2x80x9cplyxe2x80x9d, and to place a number of such plies on top of one another such that fibers in one ply are skew with respect to fibers in an adjacent ply. With suitable arrangements of the plies it is possible to produce quasi-isotropic sheet materials. Quasi-isotropic lay-ups of thin plies of the composite can be achieved by orienting successive plies at 0xc2x0, +45xc2x0, xe2x88x9245xc2x0 and 90xc2x0; or 0xc2x0, +60xc2x0 and xe2x88x9260xc2x0 with respect to the fiber axes. The distribution of the fiber directions, however, significantly reduces the CTE influence of the fibers (as will be illustrated later); thus, it becomes that much more difficult to produce composites that have zero or near-zero CTE""s in the dominant plane of the composite.
U.S. Pat. No. 3,807,996 to Sara teaches a carbon fiber reinforced nickel matrix composite material. Sara discloses the use of high strength, high modulus carbon fibers, as well as various geometrical arrangements of the fibers, such as arrays (plates) of parallel fibers and cross-plies (laminates) of such arrays.
U.S. Pat. No. 4,083,719 to Arakawa discloses a carbon fiber reinforced copper composite featuring a low thermal expansion coefficient and no directional characteristic of the mechanical properties.
U.S. Pat. No. 4,157,409 to Levitt et al. discloses treating carbon fibers with molten NaK to permit wetting by molten aluminum, magnesium, copper, zinc, tin or lead matrix metals.
U.S. Pat. No. 5,834,115 to Weeks, Jr. et al. discloses protecting carbonaceous reinforcement materials such as fibers with molybdenum carbide and then infiltrating with a molten metal to produce a composite body. A woven fabric of coated graphite fibers reinforcing a copper matrix exhibited a CTE between about 4 and 7xc3x9710xe2x88x926 cm/cm per degree K (hereinafter conveniently referred to as xe2x80x9cparts per million per degree Kelvinxe2x80x9d or ppm/K).
High modulus carbon fibers have also been incorporated into polymeric matrices. U.S. Pat. No. 5,330,807 to Williams discloses a composite laminated tubing intended for offshore oil extraction operations. There the problem was the need to transfer oil in a tube over appreciable distances and in which the tube may undergo considerable temperature excursions due to the elevated temperature of the extracted oil. To minimize the expansion of the tube length and thereby ameliorate the propensity for the tube to fail by buckling, the tubing is made of a plurality of layers of fibers fixed in a plastic matrix. The fibers may be graphite fibers, glass fibers, ceramic fibers or polymer fibers, but in any case the fibers have a sufficiently low CTE as to impart to the tubing an overall CTE of no more than about 1.1 ppm/K, and a Poisson""s ratio near 0.5.
In many environments, however, polymer matrix composites cannot be used because of insufficient resistance to extremes of temperature, corrosion or radiation. Accordingly, some workers have used glass as the matrix material. Glass has several attractive properties for these types of materials, including fluidity or flowability, wettability to the fibers and the potential for relatively low CTE""s. For a laser mirror application, for example, Stalcup et al. (U.S. Pat. No. 4,451,118) hot pressed a mixture of a low expansion borosilicate glass and alternating plies of high modulus graphite fibers. Some of the graphite fibers were arranged perpendicular to the reflecting surface so as to be better able to conduct heat away from the mirror surface. Still, cooling passages had to be placed into the mirror structure to permit circulation of a heat exchange fluid. Similarly, in U.S. Pat. No. 4,791,076 Leggett et al. discloses a graphite fiber silica matrix composite composition having a near-zero overall CTE. In addition to silica, the matrix contains boron phosphate and beta-spodumene, and Leggett states that the composite CTE is tailorable between xe2x88x921 and +1 ppm/K by varying the matrix composition. As a consequence of the low CTE, very little thermal distortion occurred in for example, a laser mirror application, particularly at low coolant flow rates. This glass matrix composite material exhibited much less thermal distortion than did other laser mirror materials such as single crystal molybdenum or silicon. Although the cooling requirements were reduced, active cooling techniques involving heat transfer media flowing through channels in the mirror still were required.
As mentioned above, glass matrix composites have been used in environments where low expansion polymer composites would be insufficiently durable. Many of these applications, however, require high thermal conductivity, and most glasses are deficient in this area. Thus, composites workers have attempted to address the thermal conductivity problem by relying on the carbon fibers to carry this responsibility, the carbon fibers possessing relatively high thermal conductivity in the fiber axis direction. Another problem with glass matrix composites, though, is that they tend to be brittle. In many applications in which such composites are subjected to accelerations and stresses, such as with semiconductor fabrication equipment, it would be preferable to have a tougher, more impact resistant material.
A number of metals are intrinsically highly thermally conductive and tough, and possessing low specific gravity and sufficient durability in harsh environments as to make them candidates for aerospace or precision equipment applications. Unfortunately, these metals suffer from having relatively high CTE""sxe2x80x94typically around 20 ppm/K or higher. There seem to be no successes or even proposals to make composites using these high modulus carbon fibers as the reinforcement of a light metallic matrix for the express purpose of producing very low CTE metal matrix composites. The lowest CTE achieved for such MMC""s appears to be the 4 ppm/K of U.S. Pat. No. 4,083,719, which represents work done years ago. While quite low in comparison to unreinforced metals, there are applications, such as in optical systems that undergo temperature fluctuations, where even lower CTE""s would be desirable.
Thus, in view of the present state of composites development, it is an object of the present invention to produce a metal matrix composite material that exhibits a low CTE, and particularly a near-zero CTE, preferably in a quasi-isotropic condition.
It is an object of the present invention to produce a material having a relatively high thermal conductivity.
It is an object of this invention to produce a material that can maintain its structural integrity at higher temperatures than can polymeric materials.
It is an object of this invention to produce a material that is more resistant to the effects of radiation than are polymers.
It is an object of this invention to produce low expansion composite materials that are tougher and/or more impact resistant that glass matrix composites.
These and other objects of the present invention are achieved by reinforcing a lightweight metal or semimetal such as aluminum, magnesium, copper, silicon or one of their alloys, with a fibrous reinforcement having a negative CTE in the axial direction. Because the CTE of the matrix is positive while that of the fibers is negative, the individual contributing CTE""s tend to cancel one another. Because of this counterbalancing or offset effect, it is theoretically possible to engineer a metal matrix composite material, or a composite material of any matrix, for that matter, to have a net overall CTE of zero.
The inventors recognize, however, that the CTE of the composite body is also influenced by the elastic modulus of the individual composite constituents. Given the high CTE and elastic modulus of some candidate matrix metals, it is for all practical purposes impossible to achieve sufficiently high fiber loadings to counterbalance this xe2x80x9cadverse influencexe2x80x9d of such matrix metals, in spite of the fact that the negative CTE fibers usually also possess very high elastic modulus.
Significantly, the inventors appreciate that the lower the modulus of the matrix relative to the fibers, the more that the CTE of the composite is influenced by the CTE of the fibers. Accordingly, by introducing voids or porosity into the matrix, the elastic modulus of the matrix may be decreased, thereby shifting the composite CTE further toward that of the fibers. The important point is that the direction of the change in composite CTE is toward zero. One such technique for introducing such porosity is by dissolving hydrogen gas into a melt of the metal that is to be the matrix, and causing the gas to come out of solution as bubbles upon solidification of the metallic matrix. Another is to incorporate one or more filler materials comprising hollow bodies into the composite.